1. Field of the Invention
The present invention relates to a friction clutch, in particular for a motor vehicle, and more particularly it relates to a clutch equipped with a device for compensating for wear, particularly in at least one friction liner, referred to hereinafter as a wear take-up device.
2. Description of Related Art
A conventional friction clutch generally includes a reaction plate, which may be in two parts so as to constitute a damping flywheel or a flexible flywheel, which is mounted in rotation on a first shaft, usually a driving shaft such as a crankshaft of the internal combustion engine, and which supports through its outer periphery a cover plate to which at least one pressure plate is attached.
The pressure plate is fixed with respect to the cover plate and reaction plate for rotation therewith, while being displaceable axially under the biasing action of axially acting resilient clutch engaging means controlled by declutching means; the clutch engaging means may consist of helical springs, or one or two Belleville rings mounted in series or in parallel and subjected to the action of declutching levers which constitute the declutching means; generally, the clutch engaging means and declutching means are part of the same component, for example a metallic diaphragm bearing on the cover plate; the diaphragm may be mounted in series or in parallel with a Belleville ring so as to assist-the declutching force, in the manner described for example in the document FR-A-2 753 758.
A friction disc, carrying one or more friction liners at its outer periphery, and rotatable with a shaft, usually a driven shaft such as the input shaft of the gearbox, is interposed between the pressure plate and reaction plate so as to be gripped between them when the clutch is engaged. The clutch engaging means control the axial displacement of the pressure plate when actuated by a clutch release bearing through the declutching means.
In the course of the useful life of such a clutch, the friction liner or liners and the materials of the cooperating components, namely the pressure plate and reaction plate, become worn, which causes a variation in the position of the pressure plate and the positions of the axially acting clutch engagement means and clutch release bearing, as a result of which there is, firstly, a variation in the force with which the friction disc is gripped between the pressure plate and reaction plate due to the modifications in the working conditions of the clutch engaging means, and secondly, the force necessary to disengage the clutch is affected. By giving such a clutch a wear take-up device, these disadvantages are avoided since the clutch engaging means and the clutch release bearing, which is usually in constant engagement on the declutching means, occupy the same position when the clutch is engaged.
A friction clutch equipped with a wear take-up device is arranged in such a w ay that one of the abutments of the clutch engaging means, whether the abutment is coupled directly or indirectly to the cover plate or whether the abutment is coupled directly or indirectly to the pressure plate, is displaceable axially, and a so-called compensating means is provided for performing this displacement in association with a so-called actuating means which starts, and in fact actuates, the said compensating means, and a triggering means which itself controls the actuating means by authorising or not authorising the starting of the actuating means. A wear take-up device is of course governed by a so-called detection means which is responsive to wear in the liner or liners, directly or indirectly, that is to say it is for example responsive to the position of at least one of the components of the clutch influenced by the said wear, with the said detection means acting on one of the said means consisting of the compensating means, actuating means and triggering means.
Various types of wear take-up device are known.
For example, document FR-A-2 753 503 describes a clutch in which the wear take-up device comprises ramp means disposed circumferentially and constituting the compensating means, being placed axially between the displaceable abutment and the pressure plate and being adapted to be driven in rotation by a set of teeth which they carry at their periphery and with which a tangentially disposed worm cooperates, means being provided for driving the worm in rotation and being rendered operational by wear in the friction liner or liners, together with non-return means that prevent the worm from rotating in the opposite direction from that in which it is driven in rotation by the rotational driving means when these are operational, the worm, the non-return means and the rotational driving means for the worm being carried by a support which is fixed to the cover plate, and the axially acting resilient clutch engaging means consisting of a diaphragm. The means for driving the worm in rotation are controlled by the diaphragm; the ramp means consist of an annular ring which has the displaceable abutment for the diaphragm and ramps which are inclined and spaced apart circumferentially, while the pressure plate has pads or ramps which are adapted to cooperate with the ramps of the ramp means; the worm is mounted for sliding movement on its axis, being subjected to the action of a so-called wear take-up resilient means constituting the actuating means, and when it is displaced on its axis, it drives the said ramp means in circumferential rotation. The means for driving the worm in rotation consist of a ratchet wheel fixed to the worm, and the non-return means consist of a pawl that cooperates with the ratchet wheel. In this case the detecting means is a tongue which is actuated by the diaphragm and adapted to cooperate with the ratchet wheel, with its course of travel increasing with wear until, with the clutch engaged, it can push on one tooth of the ratchet wheel and so cause the worm to turn, thereby compressing the elastic wear take-up means; the triggering means is represented by a reduction in the load on the band of the elastic wear compensating means on which engagement takes place during the declutching operation, when its load is sufficiently high to overcome the friction forces, thereby enabling the inclined ramps to be displaced and therefore enabling the plate abutment to be displaced axially.
Thus, the detection means, in this case the tongue controlled by the diaphragm, acts via the ratchet wheel on the actuating means which here comprise the resilient wear take up means, thus causing an actuating force to appear; triggering takes place during a subsequent declutching operation.
In the document GB-A-2 294 983, the clutch engaging means consist of a diaphragm and the displaceable abutment is again the plate abutment; the compensating means again consists of the ramps; the actuating means is a spring which acts permanently between ramps and counter ramps in the direction of increase of the axial thickness; here again, the triggering action takes place during the declutching operation by reduction in the load on the band of engagement, while the pressure plate is displaced as a function of wear with respect to a pin in engagement on the reaction plate, with the said pin, which projects frictionally through the pressure plate, constituting the detection means. This pin carries an abutment in the form of a lever that cooperates with the ramp means; the detection means then acts on the compensating means.
In the document GB-A-2 294 301, the clutch engaging means once again consist of a diaphragm, the means for compensating by means of ramps and the spring actuating means acting permanently between the ramps and, in this case, the cover plate; in this case the displaceable abutment is the primary abutment carried by the cover plate; the clutch is of the push-to-release type, and triggering is obtained by lowering of the secondary abutment which is situated facing the primary abutment on the same side as the pressure plate and mounted elastically, while the maximum declutching load increases with wear and this constitutes- the detection means, the descent of which constitutes the triggering means. The detection means accordingly acts on the triggering means, and a clearance appears between the primary and secondary abutments as the secondary abutment descends, the said clearance being taken up by a movement of the primary abutment under the action of the actuating means.
In the document U.S. Pat. No. 5,564,541, in the version shown in FIG. 10, the detection means is a pin which is disposed with a clearance between the cover plate and the reaction plate and which extends frictionally through the pressure plate, the force necessary for displacement of the pin with respect to the pressure plate being greater than the return force of the pressure plate which in this case is due to the tangential tongues; the actuating means is a spring that acts permanently between the pressure plate and the ramps that constitute the compensating means; it is the plate abutment which is displaceable, the clutch as a whole being of the pull-to-release type. In this case the course of travel for disengaging the clutch varies: the fully declutched position is fixed in such a way that triggering takes place at the end of this declutching operation, after wear has been detected and a clearance has appeared, this clearance being subsequently filled under the action of the actuating means.
In the document FR-A-2 599 446, in the version in FIG. 5, the elements are analogous to those in the preceding document, except that the compensating means is a free wheel working in only one axial direction, and the actuating means is a spring which permanently biases the balls into engagement against their ramps.
In the version in FIG. 3 of the document U.S. Pat. No. 5,564,541, the detection means is a pin which is disposed with a clearance between the cover plate and the pressure plate and which extends frictionally through the cover plate, the force necessary for displacing the pin with respect to the cover plate being greater than the return force of the pressure plate, which in this case is due to the tangential tongues; the actuating means is a spring which acts between the cover plate and an annular rotary ring having ramps adapted to cooperate with counter ramps; it is therefore the cover plate abutment which is displaceable, the clutch being of the pull-to-release type. The triggering action takes place at the end of the declutching operation, after wear has been detected and a clearance has appeared, the clearance being taken up under the action of the actuating means; as in FIG. 10, the position of full declutching is fixed, the clutch engaging means comprising declutching levers associated with helical springs which bear on the cover plate.
When a clutch is equipped with a wear take-up device which operates automatically, it is relevant for the user to have some idea of the degree of wear in the friction liners; in this connection, without this information he runs the risk of using his friction disc until the friction liners are substantially worn out, and this situation can lead to serious damage to the friction surfaces of the flywheel and/or the pressure plate, which are expensive to replace; this is the case for example when, the friction liners of the friction disc being held in place by rivets, the latter start to rub against the pressure plate and to damage the friction surface of that plate.
It has in the past been proposed, for example in the document FR-A-2 496 204, to monitor the position of the friction disc which is displaced axially as a function of wear, for the information of the user; such an arrangement makes use of induction coils and requires space to be available in the centre and increases the axial size of the clutch.
Given that the control members for the clutch engaging means also shift as a function of wear, it has been proposed to measure this shift, especially in the documents U.S. Pat. No. 3,982,614 or FR-A-2 712 947; not only are solutions of this kind not simple, but above all they have the disadvantage that they cannot be applied to a clutch equipped with a wear take-up device.
In the French patent application filed on Jun. 17, 1998 under the No. 98 07639, the wear take-up device itself is made use of so as to inform the user of the state of wear in the friction liners; in this case, it includes circuit breaking means for putting it out of circuit before the liners are fully worn; as wear continues, the position of the diaphragm changes in the engaged state of the clutch, and therefore its load and the force needed for declutching increases, which warns the user that he has passed the maximum safe value of wear, for example 80%.
Such an arrangement operates well; it does however suppose that the user has some familiarity with the driving of his vehicle.
An object of the invention is to overcome these drawbacks, while also taking advantage of the wear take-up device itself, and more precisely its operation, for the purpose of warning the user of a certain degree of wear, by creating a sharp break in the progress of the wear take-up operation which is adapted to modify, rapidly and significantly, the position of the control members for the declutching means; thus, the user is very clearly warned, regardless of his state of familiarity.
Thus, according to the invention, a friction clutch, in particular for a motor vehicle, of the kind comprising a reaction plate designed to be mounted in rotation on a driving shaft, a friction disc carrying at its outer periphery at least one friction liner, adapted to be mounted in rotation on a driven shaft, a pressure plate, a cover plate fixed on the reaction plate, axially acting clutch engagement means which are controlled by declutching means and which act between, firstly, the cover plate and, secondly, the pressure plate through interposed abutment means, the pressure plate being fixed to the cover plate for rotation with the latter while being displaceable axially with respect to it and being subjected to the action of elastic return means biasing the pressure plate axially towards the cover plate, the said clutch also including a wear take-up device, one of the abutments of the clutch engaging means being displaceable axially and the wear take-up device including a compensating means actuated by an actuating means and arranged to effect the said displacement, the said actuating means being controlled by a triggering means in association with a detection means responsive to wear in the friction liner and acting on at least one of the actuating means, compensating means and triggering means, is characterised by the fact that the wear take-up device further includes so-called divergence means carried by one of the elements in the group consisting of the compensating means, actuating means, triggering means and detection means, adapted to modify rapidly, for a predetermined amount of wear in the friction liner, the position of the control members for the declutching means.
The change is felt by the user and/or can be detected in the control of the declutching means.
The predetermined amount of wear may be the total or substantially the total wear of the friction liner or liners. In one embodiment, the rapid change is an abrupt change.
Further features of the invention are defined in the dependent Claims at the end of the description.